1995 Chevy P30 7.4l Tbi

ddavison · December 9, 2007

This is also posted in the hd truck forum

I have a 1995 chevy p30 18' box truck with a 7.4L TBI and I am experiencing some strange problems.

The truck's fuel pump went bad and was replaced along with the inline fuel filter and a new tank strainer and after replacing, the truck started but idle was cycling up and down and when I put the truck in gear, it would stall.

When I started the engine again and reved the engine slightly I would get small but rapid backfiring thru the throttle body

so I did some checking and it was suggested that I replace the fuel pump pulsator with a short piece of fuel line that comes with the fuel pump to see if that cures the problem. I did this and found no change.

I checked for vacume leaks and found no leaks anywhere and so I moved to the next step and pulled the codes and got code 54. but since this engine has a tbi the code did not make any sence.

I checked the cap and rotor which is fairly new and they both checked out ok. Now just to be safe I pulled the distributer and replaced the pick-up coil and the control module. Set the base timing to 4 degrees btdc with the single control wire unpluged as per the manuals procedure. I then cleared all codes in the ECM by disconnecting power and waiting 60 minutes (lunch hour)...

came back from lunch and reconnected the power and started the engine....engine started, idle cycled up and down and smoothed out after pedeling the throttle for a few minutes and then the idle went from normal to a fast idle and stayed that way for about 10 minuted and then I reved the engine for a moment and idle returned back to normal and then it would cycle up and down and then smooth out ....cycle a bit then smooth out.... turned of engine and pulled codes again ....got a code 35 (idle air control)

Started engine again ...idle normal ...put truck in gear ....stall.....start engine again ...put in gear again but peddled throttle to keep engine running.....engine surged and lobed and bucked.....pulled truck back into the shop and let idle. hmmmm fuel issue....ok fine grabbed a painters clamp light with a 100 watt bulb and looked at the two injectors spraying ....pattern looked fine but I noticed that if I moved the throttle to the wide open position ...the injectors stoped spraying for a moment and then started again but engine bogged as if it were leaning out and as this was happening I noticed rapid poping or backfiring up thru the throttle body at medium to high RPM....interesting

Ok... heres what I have to work with

#1... IAC

#2... Linear EGR

#3... TPS

#4... MAP

#5... ECM

Since I know that the throttle actuater is working and the fact that my main problem seems to be bogging under any kind of load, Im Going to try replacing the TPS and the MAP on this next round then if that does not cure the problem I will move on to the IAC and the linear EGR. I have a new ECM but that will only get replaced as a last resort.

Now heres something I find Interesting ... When the truck idles smooth or cycling, I noticed that the exaust smells like that of an engine dieseling or suffering from run on after the ignition is turned off .....and it really makes your eyes water and irratated.

I am quite sorry for this thread being so long winded but I thought it would help if I brought the group up to speed step by step as to the problems Ive experienced .

Thankyou for your help

Denis

[email protected]

ddavison · December 11, 2007

This is also posted in the hd truck forum

I have a 1995 chevy p30 18' box truck with a 7.4L TBI and I am experiencing some strange problems.

The truck's fuel pump went bad and was replaced along with the inline fuel filter and a new tank strainer and after replacing, the truck started but idle was cycling up and down and when I put the truck in gear, it would stall.

When I started the engine again and reved the engine slightly I would get small but rapid backfiring thru the throttle body

so I did some checking and it was suggested that I replace the fuel pump pulsator with a short piece of fuel line that comes with the fuel pump to see if that cures the problem. I did this and found no change.

I checked for vacume leaks and found no leaks anywhere and so I moved to the next step and pulled the codes and got code 54. but since this engine has a tbi the code did not make any sence.

I checked the cap and rotor which is fairly new and they both checked out ok. Now just to be safe I pulled the distributer and replaced the pick-up coil and the control module. Set the base timing to 4 degrees btdc with the single control wire unpluged as per the manuals procedure. I then cleared all codes in the ECM by disconnecting power and waiting 60 minutes (lunch hour)...

came back from lunch and reconnected the power and started the engine....engine started, idle cycled up and down and smoothed out after pedeling the throttle for a few minutes and then the idle went from normal to a fast idle and stayed that way for about 10 minuted and then I reved the engine for a moment and idle returned back to normal and then it would cycle up and down and then smooth out ....cycle a bit then smooth out.... turned of engine and pulled codes again ....got a code 35 (idle air control)

Started engine again ...idle normal ...put truck in gear ....stall.....start engine again ...put in gear again but peddled throttle to keep engine running.....engine surged and lobed and bucked.....pulled truck back into the shop and let idle. hmmmm fuel issue....ok fine grabbed a painters clamp light with a 100 watt bulb and looked at the two injectors spraying ....pattern looked fine but I noticed that if I moved the throttle to the wide open position ...the injectors stoped spraying for a moment and then started again but engine bogged as if it were leaning out and as this was happening I noticed rapid poping or backfiring up thru the throttle body at medium to high RPM....interesting

Ok... heres what I have to work with

#1... IAC

#2... Linear EGR

#3... TPS

#4... MAP

#5... ECM

Since I know that the throttle actuater is working and the fact that my main problem seems to be bogging under any kind of load, Im Going to try replacing the TPS and the MAP on this next round then if that does not cure the problem I will move on to the IAC and the linear EGR. I have a new ECM but that will only get replaced as a last resort.

Now heres something I find Interesting ... When the truck idles smooth or cycling, I noticed that the exaust smells like that of an engine dieseling or suffering from run on after the ignition is turned off .....and it really makes your eyes water and irratated.

I am quite sorry for this thread being so long winded but I thought it would help if I brought the group up to speed step by step as to the problems Ive experienced .

Thankyou for your help

Denis

[email protected]

UPDATE! Ok, while doing some testing, the new waterpump that I installed 2 years ago failed and because it is behind every accessory bracket that has to come off to R & R the waterpump, I figured why not put a timing chain in since the engine has 165,000 miles on it. Now when it comes to timing chains I go straight for the double roller type every time. they only cost ten bucks more that the stock style and they are a lot more durable.

Denis

awalkertx · December 12, 2007

hi dennis -

on the smell thing - what's base timing (im not familiar with bb)? 4btdc on my tbi sb 350 would be 4 degrees advanced... might be retarded for you. if theyre retarded they start to smell funny. that doesn't sound politically correct!

as far as the IAC valve, it fixed my truck's idling problem... which seem similar to yours. i'd go with that $45 part before anything else at least to fix the idling issue.

when you say that you push the butterfly open to WOT, there's a pause and a break in the fuel flow? possible fuel pump problem? i know it doesnt need much pressure for tbi, but maybe there's a volume issue?

ddavison · December 13, 2007

hi dennis -

on the smell thing - what's base timing (im not familiar with bb)? 4btdc on my tbi sb 350 would be 4 degrees advanced... might be retarded for you. if theyre retarded they start to smell funny. that doesn't sound politically correct!

as far as the IAC valve, it fixed my truck's idling problem... which seem similar to yours. i'd go with that $45 part before anything else at least to fix the idling issue.

when you say that you push the butterfly open to WOT, there's a pause and a break in the fuel flow? possible fuel pump problem? i know it doesnt need much pressure for tbi, but maybe there's a volume issue?

BTDC = before top dead center. ATDC = after top dead center. my engine call's for 4 degrees before top dead center and once this static timing is set, the computer controls the spark advance based on the information that it gets from the sensors. I put a new fuel pump in it already along with a new inline filter. the poping started prior to the fuel pump going bad. I dont think this is a timing issue, but rather a fuel signal issue. From what ive seen, the ECM cant control the fuel curve or is loosing the signals that it needs to control the fuel curve. If I take and do a hard acceleration of the engine, somewhere in the middle of the power band before the rev limiter kick in, the injectors shut down for a brief moment and the engine backfires thru the top of the TBI due to a severe leanout condition. the big 64 dollor question is which sensor or combination of sensors arent sending the correct information to the ECM. it could be the ecm itself or it could be a bad connection on one of the sensors plugs. Im not the only one to be experiencing these problems. their are a lot of people who own p-30 chassies be it a motor home or in my case a box truck that are suffering from these same exact issues. the worst part is that the dealership mechnics who are supposed to know all the answers dont have a clue as to whats wrong. all they know how to do is replace parts untill the problem is corrected....all at the customers expence. these problems arent just with the p-30 chassie, their with all the models that have small blocks or big blocks. its just really poor design compounded by over-engineering. this is the exact reason why the the big 3 are loosing their ass to foreign manufacturers. the american car companys over complicate everything rather than just keeping it simple. take for instance the throttle system on the new cars. A $10.00 throttle cable that operates a simple butterfly on the intake has been replaced by an $250.00 electronic servo motor that takes a throttle pedal sensor and a computer to do the same job. all this bulls**t does is drive the over all price of the car or truck sky high and make it damm near impossible to repair without spending thousands of dollors on parts and labor.

Denis

ddavison · December 20, 2007

Ok ....I have the truck back together and I fired it up and it ran fine. It went thru the high idle step untill it warmed up and then it started doing a hi/low cycling and then it died. I rechecked all the vacume lines one by one and when I disconected the vacume line to the map it idled fine. when I reconected the vacume line to the map.....it stalls

any thoughts?

Denis